Wind turbines are used for converting the power of wind to a source of energy, typically electrical energy. In recent years, the use of yet larger wind turbines has increased the requirements of the connection between the wind turbine blades and the flange of the wind turbine hub. A critical feature is the connection between the fastening members for securing the blade to the hub, which fastening members are provided at the blade root of the blade and the rest of the blade as this connection is subjected to large and cyclic loads during operation.
A durable and yet affordable wind turbine blade manufactured by combining pre-fabricated sticks by vacuum-assisted resin infusion is disclosed in WO 03/008800.
In U.S. Pat. No. 4,915,590 a number of techniques for connecting a wind turbine blade to a hub is described. Particularly the prior art technique described in relation to FIG. 3 of U.S. Pat. No. 4,915,590 is relevant in relation to the present invention. There, steel attachment stud rods are embedded in a wind turbine root by drilling a hole into the end of the wall of the completed wind turbine blade and fixing the attachment stud rods in the hole by adhesive. Drilling of holes into the laminated composite structure is very time-consuming, laminate material is wasted and the wear of the drill is considerable. Furthermore, removal of debris from the hole as well as preventing presence of air in the adhesive is complicated due to the length to width ratio of the hole. A considerable space between the attachment stud rod and the walls of the hole is therefore required leading to a large consumption of adhesive, a relatively weak connection and an unduly large blade wall thickness.
It has been considered to provide the fastening members prior to curing of the blade. WO 03/082551 discloses a blade connection wherein an insert is inserted into a non-cured laminate structure. The insert is provided with a plurality of elevations and recesses of similar longitudinal size and smaller pieces of parallel laminate layers are provided in the recesses. It is necessary to sew the laminate layers together near the insert to provide sufficient de-lamination strength between the smaller pieces of parallel laminate layers and larger outer laminate layers. Sewing of layers is a very time-consuming and complex process and must either take place from the inner side of the hole or from the outer side during lay-up of the laminate layers. Sewing may also interact with the alignment of fibres leading to a decrease in strength of the laminated structure at the most critical place, namely near the inserts.
There is hence a need for a fast and secure method for providing a fastening member in a wind turbine blade.